Darth Vader (R) is quickly became one of the most sought after cards when it was realeased in the Revenge of the Sith set, and for good reason. Because of it's Hidden Cost 5 effect, it can easily be deployed, and it has enough power to do significant damage to other characters, and the best part is: That damage can't be prevented, at all.
I find Vader most useful for countering Jedi, especially more powerful ones like Yoda (C) and Mace Windu. The advantage the Jedi have over other characters (Evade and Deflect) is null against Vader, which makes him extremely effective against them. His deflect ability is also extremely useful, because it can't be prevented either. With just 4 force, you can do up to 4 damage to any character, damage that can't be stopped. That make s Vader one of the only cards capable of destroying a card like Yoda, Mace Windu, and other powerful Jedi.
Strategies and game play:
Vader is most effective when stacked, because once his speed is increased, even by just 10, he is almost always the first to attack, which means he has a chance to destroy one of your opponents characters before they even get a chance to fight. Stacking also gives him more power, and for Vader, power is essential. Because his damage can't be prevented, you should power him up as much as possible with battle cards (Surge of Power is always good), and again: stack him with other Vader cards.
His hidden cost ability also makes him extremely easy to deploy, unlike other Sith such as Darth Tyranus (A) and Darth Maul. If you have enough force, you could send him into battle for just 5 build points. You have to be careful with your force though, because you'll need it in order to prevent damage to Vader (as well as your other cards). Keeping him alive is essential, and he should be considered priority to your other characters.
His last effects, Deflect and Evade, can be put to good use. Especially the deflect, because that counts as damage from Vader, which means it can't be prevented.
Combos with other cards:
It makes some nice combos with cards such as Surge of Power, and other battle cards that increase his speed or give him critical hit. I actually find him most effective with just him and another Sith in the Character arena, otherwise your force will be used up quickly. Something such as Darth maul (a) is great, because of his speed, meaning he can weaken a character before Vader gets to attack.
Ways to counteract it:
The card I've found most effective against Vader is Yoda (C), because of his ability to do 4 damage automatically. That means in just two turns, you can have Vader off the field. Otherwise, I'd would suggest concentrating power on him, and force your opponent to use as much force as possible.
The biggest strength for Vader is definitlely that his damage can't be prevented. That takes away the entire advantage of Jedi, which is typically the main type of light side character. His hidden cost also allows him to be deployed easily, especially if you have some force to spare.
Well...there aren't many. Having to pay 3 force to evade 2 is a disadvantage, because you typically want the amount of force you pay to be equal to the damage you'll prevent, but his Pay 1 Force > Deflect 2 somewhat makes up for it.
As far as I am aware, this card only comes in the ROTS packs. It can be in foil though.
Artwork and aesthetics:
The picture for this card couldn't be more fitting. It shows Anakin at the height of his evil in ROTS, and the way he's staring makes it look like he's staring right into you. Pretty awesome in short.
I'm gonna give him a 9/10. This card has one of the best effects in the game, and just one small weakness, but that weakness could be exploited. Nonetheless, it's definitely one of, if not the best card in the game.